Whisky Basics 4/4 – Maturation

by Oliver Klimek on July 25, 2009

This is the last part of a series of four articles about the basic properties of whisky. The other parts dealt with whisky types,regional variations. and the production process. In this part I want to outline the influence of maturation on the whisky.

1. How whisky is stored

After distillation, whisky is filled into wooden barrels and left there to mature for several years. Some countries have specific legislation about whisky storage, especially the traditional whisky producers like Scotland or the USA. The Barrels or casks are stored in warehouses that are usually open to make the exchange of air easier.

2. Legal requirements

2.1 Scotland

Scotch whisky has to be stored in Scotland in oak casks with a maximum size of 700 litres for at least three years. There is no further specification for the cask.

2.2. Ireland

Irish whiskey has to be stored in Ireland or Northern Irleland [!] in wooden casks for at least three years. That’s all.

2.3 USA

American regulations are dependent of the type of whiskey. All have in common a minimum storage of two years. Bourbon, Rye and Straight (without naming a grain) has to be matured in new and charred oak barrels. Corn whiskey does not have to be aged but, if it is aged, it must be in new un-charred oak barrels or used barrels. Tennessee whiskey may also use used barrels.

2.4 Canada

Canadian whisky has to be stored in Canada in wooden casks with a maximum size of 700 litres for at least three years.

One interesting thing that you can notice here is that Scotland and USA require casks to be made from oak, whereas in Ireland and Canada any kind of wood may be used. But oak is generally considered to be the best cask material for any spirits.

3. What happens in a cask during maturation ?

During maturation in a cask, whisky leeches aromatic components from the wood by diffusion because alcohol is a good solvent for many aromatics. The smaller the cask, the more influence the wood has because the ration of surface to volume is bigger. In the case of a used cask, also residues from liquids that were contained in the cask before diffuse into the whisky.

4. New or used Casks ?

As just noted, most US whiskeys have to be matured in fresh casks. Although in Scotland there are no regulations on what type of oak cask can be used, Scotch whisky has always been matured in used casks (only very recently some distilleries started to experiment with fresh casks as well). What is the reason for this difference?

Other than Scotch, US whiskey has only a small fraction of malted barley in is ingredients list. The other main components, corn or rye, are used unmalted. Basically this is because barley is the best grain for malting but corn and rye are more popular crops in America. As pointed out in part 3, malting adds flavour because of the Maillard reactions.

Charred Whiskey Barrels

To compensate this deficiency, Maillard flavour components are brought in by charring the inside of the barrel. Obviously charring creates a layer of charcoal, but in the regions of the wood where it is not hot enough for the creation of charcoal, the heat is sufficient to split up the cellulose into sugars to induce the famous Maillard reactions. In addition to that, sugars already present the wood are caramelized. The main flavour components created by charring are vanilline and caramel.

Charring is only done with fresh barrels, as it would destroy any other liquid confined in the pores of the wood of a used barrel anyway.

Because a fresh oak barrel is very active in giving away aromatic components, maturation times for Bourbon etc. are relatively short. This is also indicated by the legal US minimum storage time of only two years compared to the three years minimum for other regions.

Malt whisky being inherently more flavourful is usually aged in used casks. Here the emphasis lies on the influence of the previously contained liquid instead of the wood itself. But one must not forget that that liquid has also been influenced by the wood for quite some time. So maturation in a used cask will always be different from adding that liquid directly (the Canadian way, so to speak [see part 2]).

4. What used casks to use for maturation ?

4.1 Sherry

Traditionally, sherry casks have been a favourite for maturing Scotch whisky. Britain has always been a main market for sherry, and as sherry was transported by ships in 500 litre “butts”, it was pretty logical that these casks were used again for whisky. After all, using these casks saved the work of making new ones.

Nowadays, sherry is bottled in Spain, so distilleries have switched to buying old solera casks used for sherry maturation in Spain or even make contracts with sherry producers to “season” casks with sherry for several months or even years just for the purpose of subsequent whisky storage.

The influence of sherry is very noticeable when it is a first-fill cask, i.e. whisky came in right after the sherry got out. Because casks are used over and over again the influence becomes weaker after every refill.

Sherry influence in whisky usually relates to notes of dried fruits, honey or toffee. Because there are many different types of sherry, there is a large bandwidth of aromatic components to be found in different whiskies.

4.2 Bourbon

Some distilleries refrain from using sherry casks. For one because they are not that easy to get, but also because they just don’t want to have that “sherry taste”. Another option on the cask market are used bourbon casks, or hogsheads as they are also called. As American bourbon makers always have to use fresh barrels they are quite happy that they can sell their used ones to Scottish or Irish distilleries.

Despite all the differences in taste, bourbon is also whisky, so maturing scotch in a bourbon barrel gives a much “cleaner” taste than a sherry cask. But nevertheless there is a certain influence. As noted before, mainly vanilla and caramel notes show through when a malt is aged in a bourbon hogshead.

5. Finishes

Not long ago, someone had the idea that before bottling a whisky it would be nice to to fill a it into a cask that once held something completely different than the cask used for maturation for a short while. This was called “finish”. Obviously that would give a distinctive touch to the final product.

The idea spread like a wildfire in the whisky world, and soon many distilleries and independent bottlers jumped onto the bandwagon and finished their whiskies with all kinds of casks: cognac, tokay, madeira, port, Chateau XY, you name it.

In principle, finishes are not a bad thing at all. Sometimes they really taste great and add a new dimension to a whisky. But sometimes it just seems that finishes are only used to cover flaws that would be noticed in the unfinished version. And in other cases it might turn out that the experiment just didn’t work at all and the finish in fact destroyed the whisky.

This is the end of the series about whisky basics. Just about every topic could still be further examined, and perhaps it will be in the future.